The present invention relates generally to electronic components of resin molded package type, and, more particularly, to electronic components suitable for surface mount technology.
Electronic components of the surface mount type are well known. FIG. 22 shows an external view of a conventional electronic component of the surface mount type which incorporates therein a semiconductor chip (an electronic element). Electronic component 70, shown in FIG. 22, has elongated, first to third leads 71-73. 74 is a resin having the form of a rectangular parallelepiped which encapsulates a semiconductor chip. Each lead 71-73 horizontally stretches out from opposite lateral surfaces of resin 74 and has an L-shaped bent. The bottom dimensions of resin 74 is 1.6 mm.times.0.8 mm, and each lead 71-73 projects from resin 74 for from 0.4 mm to 0.6 mm. To prevent poor soldering (i.e., defective soldering in which, when mounting electronic component 70 onto a printed board, a good bond is not produced at a junction between the printed board and each lead 71-73), it is required by lead stand-off specifications that the bottom surface of resin 74 floats 0 mm to 0.1 mm on the printed board surface.
FIG. 23 is a plan view of a conventional lead frame for use in the fabrication of electronic component 70. Lead frame 80 of FIG. 23 is formed of a rectangular metal plate that has a uniform thickness and comprises lead formation part 81 and exterior frame 82 for enclosing lead formation part 81. Bridge part 83 is provided which acts as a bridge to connect together opposite sides of exterior frame 82, and a plurality of sets of lead parts 71-73 extend from the opposite sides of exterior frame 82. These lead parts 71-73 are one-dimensionally placed at pitches ranging from 3.0 mm to 4.0 mm, running in the lengthwise direction of lead frame 80.
The fabrication of electronic component 70, that is, the assembly process thereof, comprises a die bonding process, a wire bonding process, a molding process, a deburring process, a lead finishing process, a trimming process, and a forming process. In the die bonding process, semiconductor chips are fixed on the corresponding leading ends of first lead parts 71 of lead frame 80 of FIG. 23 using a conductive adhesive. In the wire bonding process, two electrodes on each semiconductor chip are electrically connected with the leading ends of second and third lead parts 72 and 73 by fine wires of Au (gold). In the molding process, a mold assembly for transfer mold is used for integral encapsulation of the individual semiconductor chips, the Au wires, the leading ends of lead parts 71-73 with resin 74. In this molding process, a plurality of cavities, defined between an upper mold element and a lower mold element, are one-dimensionally arranged, running in the lengthwise direction of lead frame 80 and being isolated from one another. In other words, a resin is injected, from a common runner through respective gates, into each cavity. FIG. 24 shows lead frame 80 that has undergone a molding process. In the deburring process, burrs, formed by resin escaped from fine gaps defined between the mold assembly and the lead frame 80 onto lead parts 71-73, are removed. In the lead finishing process, lead parts 71-73 extending from each resin 74 are solderplated. Deburring is an indispensable process to smooth solderplating. In the trimming process, lead parts 71-73 are cut such that individual electronic components 70 are separated. In the forming process, bending is performed so that each lead part 71-73 has a bent, as shown in FIG. 22.
The above-described conventional electronic component 70 has elongated leads 71-73 which project from lateral surfaces of resin 74. Therefore, electronic component 70 occupies a considerably larger space in comparison with a semiconductor chip (an electronic element), therefore producing the problem that mounting density on a printed board cannot be increased much. Additionally, leads 71-73 which project from resin 74 are vulnerable, which may result in an unexpected change in the lead form at the time of mounting an electronic component 70 onto a printed board, thereby causing defective soldering.
The above-described fabrication method of electronic component 70 requires both a deburring process and a forming process. It is preferred that deburring is eliminated, since the deburring process produces no additional values to products. In the forming process, unexpected lead deformation and the cut of a lead may occur. Further, in the forming process, there are difficulties in always conforming to lead stand-off specifications and there has been the problem that yields tend to drop.
In the above-described lead frame 80, a plurality of sets of lead parts 71-73 are one-dimensionally arranged at a great pitch, therefore producing the problem that the number of electronic components 70 that a single lead frame 80 can yield is small. Metal material for forming lead frame 80 is used inefficiently.
In the above-described conventional mold assembly, a plurality of cavities, which are one-dimensionally arranged at a great pitch corresponding to lead frame 80, are defined between the upper mold element and the lower mold element, so that the number of electronic components 70 that a single molding process can yield is small, resulting in poor productivity.